1. Field of the Invention
The present invention relates to a regulated power supply for a laser or gas discharge lamp.
2. Description of the Prior Art
Gas lasers, such as those using mixtures of helium and neon, have certain operational characteristics that impose stringent power requirements. To turn on the laser, a relatively high voltage (typically on the order of 8kv to 10kv) must be applied across the anode and cathode. As the laser begins to ignite, the voltage across the laser decreases as the current increases. That is, the laser goes through a "negative resistance" range in which oscillation or self shut-off of the laser can occur. As more current is drawn, a positive resistance region is reached in which the voltage increases with increasing current. Stable performance requires operation in this region.
During operation changes may occur in laser performance, such as variation in the gas excitation level with concomitant change in light output. To maintain a constant light level from the ignited laser requires dynamic control of the power supplied to the laser. For example, if the light level should drop, with a corresponding decrease in current drawn by the laser, it is advantageous to increase the supplied voltage so as to bring the current level and hence the light output back up to the desired value.
To meet these requirements, the power supply initially must be able to supply a voltage great enough to cause lamp ignition. To accomplish this in the past, power supplies have been configured to provide the necessary high voltage output. A problem occurred if for some reason the laser did not ignite, or the supply accidently was turned on without the laser being connected. In this instance, the power supply continued to put out the high excitation voltage which was in excess of the continuous operating characteristics of the power supply. As a result, overheating occurred and components broke down. In contrast, it is an object of the present invention to provide a power supply for a laser in which the output voltage level automatically is decreased if the laser or other load does not ignite within a preset time duration. The power supply can operate continuously at the reduced output voltage level without overheating or other damage to circuit component.
Another difficulty with prior art power supplies concerned the prevention of operation in the negative resistance region. One approach was to use a large ballast resistor. But this had the disadvantage of dissipating excessive energy once the lamp was ignited. Thus another object of the present invention is to provide a power supply which insures operation in the positive resistance region.
Feedback circuits are known for maintaining a constant current output to the laser once it has been ignited, so as to maintain constant light output. However, the current sensing circuits in such feedback loops could not tolerate a large voltage swing, thereby limiting their operational effectiveness. A further object of the present invention is to provide a current sensing circuit which can tolerate a large voltage swing and which exhibits high dynamic impedance. Still another object is to provide a feedback loop in which the output current is sensed by a series voltage regulator.